The influence of muscular lengthening on cramps

Abstract

Muscle cramps induced by voluntary contraction and by electrical stimulation of the peripheral nerve were studied electrophysiologically in 10 healthy subjects. The aim was to verify that cramps can be evoked by electrical stimulation of peripheral nerve and to clarify the physiological mechanism responsible by analyzing the effect of muscular stretching on cramps. Our results showed: (1) Cramps can be induced even after peripheral nerve block by electrical stimulation distal to the block. (2) No cramps were recorded during or following maximal voluntary contraction without muscular shortening, while 7 of 10 subjects showed a true cramp following maximal effort with shortening of the muscle. (3) Muscle stretching caused a sudden interruption of cramps induced by either voluntary contraction or electrical stimulation of the peripheral nerve, even after the induction of nerve block. (4) The lengthening state of the muscle can strongly influence the possibility of evoking cramps by electrical stimulation of nerve. Our study verifies the experimental model proposed by Lambert in 1969, emphasizing the relevance of frequency of stimulation and confirming the hypothesis that cramps are of peripheral origin. The effects of muscle stretch and lengthening on cramp interruption and development also have a peripheral mechanism.

Reliability of a novel neurostimulation method to study involuntary muscle phenomena

“…Even if the abductor hallucis motor point can easily be found in a specific portion of the muscle, interindividual variability exists in the route of motor axons: the deeper the motor axons and their terminal branches, the greater is the current amplitude needed for supramaximal stimulation. Consistent with previous reports, 4,20,32,34 in our experimental conditions the decisive element for cramp (and fasciculation) induction appeared to be the stimulation frequency: for each subject a critical stimulation frequency, below which cramps were not elicited, was identified. The physiological determinants of the association between the frequency of the delivered stimulation train and the trigger of an autonomous and repetitive excitation of the terminal branches of motor axons are not understood.…”

“…Moreover, in the present series the threshold frequency tended to be lower in cramp-prone subjects (identified on the basis of anamnestic data of cramp occurrence both at rest and during or after exercise): this could mean that the threshold frequency may estimate the individual susceptibility to cramp development. This observation, previously reported by Bertolasi et al, 4 awaits explanation.…”

“…With respect to the previously reported methods of electrical induction of muscle cramps, 4,20,32,34 our stimulation of the main muscle motor point has the considerable advantage of triggering a cramp in the muscle (or in a part of it) without the pain due to stimulation of the sensitive afferent fibers of the peripheral nerve trunk. Moreover, the main muscle motor point can easily be identified: it was consistently found just inferior and posterior to the navicular tuberosity, in keeping with a previous study.…”

“…By contrast, Lambert 20 and Bertolasi et al 4 found that cramps of the flexor hallucis brevis muscle could be induced even after peripheral nerve block by electrical stimulation distal to the block. Moreover, the demonstration that cramps can slowly change in position (a few centimeters per second) over the muscle 30 pointed toward the intramuscular terminal branches of motor axons as their source sites.…”

“…A few experimental methods to induce a cramp of the flexor hallucis brevis muscle by means of repetitive electrical stimulation of a peripheral nerve have been reported: the stimulation frequency was the determinant of whether cramp was induced and the minimum stimulation frequency capable of inducing cramp was termed "threshold frequency" (TF). 4,20,32,34 Stone et al 34 studied the reliability of the cramp TF by recording two sessions on three separate days; they demonstrated excellent intrasession [intraclass correlation coefficients (ICCs) 0.84, 0.95, 0.98] and intersession (ICC 0.96) reliability. However, the reduced tolerability due to the direct stimulation of the posterior tibial nerve at the ankle limited the applicability of these methods to the study of both patients and healthy populations.…”